The invention relates generally to circuit breakers and, more particularly, to circuit breakers employing thermal trip means.
For certain applications, Underwriter's Laboratories has established requirements calling for a circuit breaker to carry 110% of rated current continuously at an ambient temperature of 25.degree. C. without tripping. Additional requirements call for the circuit breaker to trip within two hours at 135% of rated current and within a specified shorter time at 200% rated current depending on the breaker rating. For example, a breaker with a rating of 800 amperes must trip within 14 minutes at a test current of 16000 amperes. A circuit breaker which meets these requirements when applied to alternating current circuits is described in U.S. Pat. No. 3,296,564 issued Jan. 3, 1967 to Albert R. Cellerini and assigned to the assignee of the present invention. The circuit breaker described therein utilizes an induction heating type of thermal trip mechanism to establish the desired time-current trip characteristic.
There is increasing demand for DC breakers operating at higher ratings for applications such as uninterruptible power supplies for computer systems. In lower rating circuit breakers the bimetal portion of the thermal trip unit was often connected in series with the main contacts of the circuit breaker. However, this is not practical in higher rating breakers due to the magnitude of currents involved. Accordingly, higher rating breakers have employed induction apparatus such as disclosed in the aforementioned U.S. Pat. No. 3,296,564 to reduce the amount of current flow and heat losses in the bimetal element. However, such apparatus has heretofore precluded the use of such breakers on DC circuits.
It would be desirable to provide a circuit breaker having an induction heating thermal trip mechanism which meets Underwriter's Laboratories requirements when applied to either AC or DC circuits.